Abstract [en]

Aluminum doped zinc oxide (AZO) films have been deposited using reactive high power impulse magnetron sputtering (HiPIMS) and reactive direct current (DC) magnetron sputtering from an alloyed target without thermal assistance. These films have been compared in terms of their optical, electrical and structural properties. While both DC and HiPIMS deposited films show comparable transmittance, their electrical properties are significantly improved by the HiPIMS process. The HiPIMS deposited films show a low resistivity down to the order of 10(-4) Omega cm with a good homogeneity across the substrate, making them potential candidates for electrodes in solar cells. The density of electrons reached up to 11 x 10(20) cm(-3), making ionized impurities the main scattering defects. This improvement of the film properties can be related to the specific plasma/target interactions in a HiPIMS discharge. This allows the process to take place in the transition mode and to deposit highly conductive, transparent AZO films on large surfaces at low temperature. While the overall oxygen content is above that of stoichiometric ZnO, higher localization of oxygen is found at the interfaces between crystalline domains with substoichiometric composition. (C) 2016 Elsevier B.V. All rights reserved.

Abstract [en]

Transparent conducting oxides (TCOs) are an important class of materials with many applications such as low emissivity coatings, or transparent electrodes for photovoltaics and flat panel displays. Among the possible TCO materials, Al-doped ZnO (AZO) is studied due to its relatively low cost and abundance of the raw materials. Thin films of AZO are commonly produced using physical vapour deposition techniques such as magnetron sputtering. However, there is a problem with the homogeneity of the films using reactive direct current magnetron sputtering (DCMS). This homogeneity problem can be related to the bombardment of the growing film with negative oxygen ions, that can cause additional acceptor defects and the formation of insulating secondary phases. In this work AZO films are deposited by high power impulse magnetron sputtering (HiPIMS), a technique in which high instantaneous current densities are achieved by short pulses of low duty cycle.

In the first part of this thesis, the possibility to improve the homogeneity of the deposited AZO films by using HiPIMS is demonstrated. This improvement can be related to the high instantaneous sputtering rate during the HiPIMS pulses, so the process can take place in the metal mode. This allows for a lower oxygen ion bombardment of the growing film, which can help to avoid the formation of secondary phases. Another problem of AZO is the stability of the properties in humid environments. To assess this problem, the degradation of the electrical properties after an aging procedure was investigated for films deposited by both DCMS and by HiPIMS. A method was proposed, to restore the properties of the films, using a low temperature annealing under N2 atmosphere. The improvement of the electrical properties of the films could be related to a diffusion process, where water is diffusing out of the films. Then, the influence of the substrate temperature on the properties of AZO films deposited by HiPIMS was studied. The electrical, optical and structural properties were found to improve with increasing substrate temperature up to 600 ◦C. This improvement can be mostly explained by the increase in crystalline quality and the annealing of defects. Finally, the deposition of AZO films on flexible PET substrates was investigated. The films are growing as a thick porous layer of preferentially c-axis oriented columns on top of a thin dense seed layer. The evolution of the sheet resistance of the films after bending the films with different radii was studied. There is an increase in the sheet resistance of the films with decreasing bending radius, that is less pronounced for thicker films.